Detectives have linked the murder with an earlier attack on Jan 8. In the earlier attack, in the Strawberry Hill area of south-west London, a 17-year-old was beaten on the head with a blunt instrument and awoke in hospital with injuries which have left her "extremely traumatised" and facing several reconstructive operations.

Though the injuries in the Jan 8 attack are not consistent with a hammer, and there is no DNA to provide conclusive proof of a link, police believe the incidents are alike enough to link them.

Officers believe the man used an blunt, heavy instrument other than a hammer in both cases or discarded the first weapon after the 17-year-old was attacked and then chose a hammer to attack Miss McDonnell in Hampton, south-west London.

Both assaults were at night, within a mile of each other, and in both cases the victim had her hood up and may not have heard her attacker approaching from behind. The 17-year-old was also listening to a Walkman stereo.

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Scotland Yard is alarmed at the prospect of a disturbed individual roaming suburban streets after dark and beating young girls.

Senior detectives will deploy a mixture of covert surveillance and cameras in areas where it thought the attacker may live. He is believed to be a local man.

Officers are trawling records of local mental hospitals and institutions for potential suspects and are looking for other incidents which might have involved the attacker, such as approaching or frightening women. They have found nothing in the private lives of the two teenagers to explain the attacks.